The valves in internal combustion engines are used to regulate the flow of intake and exhaust gases into the engine cylinders. The opening and closing of the intake and exhaust valves in an internal combustion engine is normally driven by one or more camshafts. Since the valves control the flow of air into the engine cylinders and exhaust out of the engine cylinders, it is crucial that they open and close at the appropriate time during each stroke of the cylinder piston. For this reason, each camshaft is driven by the crankshaft, often via a timing belt or timing chain. However, the optimal valve timing varies depends on a number of factors, such as engine load. In a traditional camshaft arrangement the valve timing is fixedly determined by the relation of the camshaft and crankshaft and therefore the timing is not optimized over the entire engine operating range, leading to impaired performance, lower fuel economy and/or greater emissions. Therefore, methods of varying the valve timing depending on engine conditions have been developed.
One such method is hydraulic variable cam phasing (hVCP). hVCP is one of the most effective strategies for improving overall engine performance by allowing continuous and broad settings for engine-valve overlap and timing. It has therefore become a commonly used technique in modern compression-ignition and spark-ignition engines.
Both oil-pressure actuated and cam torque actuated hydraulic variable cam phasers are known in the art.
The oil-pressure actuated hVCP design comprises a rotor and a stator mounted to the camshaft and cam sprocket respectively. Hydraulic oil is fed to the rotor via an oil control valve. When phasing is initiated, the oil control valve is positioned to direct oil flow either to an advance chamber formed between the rotor and stator, or a retard chamber formed between the rotor and stator. The resulting difference in oil pressure between the advance chamber and the retard chamber makes the rotor rotate relative to the stator. This either advances or retards the timing of the camshaft, depending on the chosen position of the oil control valve.
The oil control valve is a three-positional spool valve that can be positioned either centrally, i.e. co-axially with the camshaft, or remotely, i.e. as a non-rotating component of the hVCP arrangement. This oil control valve is regulated by a variable force solenoid (VFS), which is stationary in relation to the rotating cam phaser (when the oil control valve is centrally mounted). The variable force solenoid and the spool valve have three operational positions: one to provide oil to the advance chamber, one to provide oil to the retard chamber, and one to refill oil to both chambers (i.e. a holding position).
The established oil pressure actuated hVCP technology is effective in varying valve timing, but has relatively slow phasing velocities and high oil consumption. Therefore, the latest iterations of hVCP technology utilize a technique known as cam torque actuation (CTA). As the camshaft rotates the torque on the camshaft varies periodically between positive torque and negative torque in a sinusoidal manner. The exact period, magnitude and shape of the cam torque variation depends on a number of factors including the number of valves regulated by the camshaft and the engine rotation frequency. Positive torque resists cam rotation, while negative cam torque aids cam rotation. Cam torque actuated phasers utilize these periodic torque variations to rotate the rotor in the chosen direction, thereby advancing or retarding the camshaft timing. In principle they operate as “hydraulic ratchets”, allowing fluid to flow in a single direction from one chamber to the other chamber due to the torque acting on the oil in the chambers and causing periodic pressure fluctuations. The reverse direction of fluid flow is prevented by check valve. Therefore, the rotor will be rotationally shifted relative to the stator every period the torque acts in the relevant direction, but will remain stationary when the torque periodically acts in the opposite direction. In this manner, rotor can be rotated relative to the stator, and the timing of the camshaft can be advanced or retarded.
Cam torque actuation systems therefore require check valves to be placed inside the rotor in order to achieve the “hydraulic ratchet” effect. The directing of oil flow to the advance chamber, retard chamber, or both/neither (in a holding position) is typically achieved using a three-positional spool valve. This spool valve can be positioned either centrally, i.e. co-axially with the camshaft, or remotely, i.e. as a non-rotating component of the cam phasing arrangement. The three-positional spool valve is typically moved to each of the three operative positions using a variable force solenoid.
Patent application US 2008/0135004 describes a phaser including a housing, a rotor, a phaser control valve (spool) and a regulated pressure control system (RCPS). The phaser may a cam torque actuated phaser or an oil pressure activated phaser. The RPCS has a controller which provides a set point, a desired angle and a signal bases on engine parameters to a direct control pressure regulator valve. The direct control pressure regulator valve regulates a supply pressure to a control pressure. The control pressure moves the phaser control spool to one of three positions, advance, retard and null, in proportion to the pressure supplied.
There remains a need for improved cam timing phaser arrangements. In particular, there remains a need for cam timing phaser arrangements that are suitable for use commercial vehicles, which are often subject to heavier engine loads and longer service lives as compared to passenger cars.